1. Field of the Invention
The present invention relates to an electric power steering device.
2. Related Background Art
In the field of electric power steering devices for a vehicle, there is known a configuration in which the rotary output of an electric motor, providing auxiliary steering torque, is reduced in speed by means of gears, thereby transmitting appropriate torque and speed to the output shaft of the steering mechanism. In such electric power steering device, while the device is in action, an abrupt force received by the wheels from the road may act as an impact force against the output of the electric motor, and such impact force may cause damage in the electric motor or in the power transmission mechanism.
For reducing or avoiding such impact force, there have conventionally been adopted certain measures such as:
1) a slidable mechanism utilizing frictional force, provided on the output shaft of the steering mechanism and serving as a torque limiter causing a sliding motion under an excessive torque (Japanese Utility Model Application No. 4-75506); or PA1 2) a magnetic clutch serving as a torque limiter, provided on the rotary shaft. PA1 in the structure 1) mentioned above, as the output of the electric motor is transmitted to the output shaft ordinarily through a reducing (torque increasing) mechanism, the frictional force utilized in the slidable mechanism attached to the output shaft has to be made considerably large in order to transmit thus increased torque. For this reason it is associated with a drawback of requiring a strict load control in order to obtain a constant frictional force; and PA1 in the structure 2) mentioned above, the magnetic clutch requires constant supply of electric power. For this reason it is associated with drawbacks in terms of energy saving, and the solenoid of the magnetic clutch has to be made considerably large if the rated output of the electric motor becomes larger. PA1 a housing; PA1 a motor mounted on said housing and having a rotary shaft; PA1 an output shaft connected to a steering mechanism for steering wheels; PA1 a reducing mechanism provided between said rotary shaft and said output shaft; and PA1 power transmission means provided between the rotary shaft of said motor and said reducing mechanism and adapted to transmit the power; PA1 wherein said power transmission means includes a frictional face and pressure means pressed to said frictional face and is adapted to effect power transmission utilizing a frictional force generated between said frictional face and said pressure means, wherein wherein said frictional force is so selected as to be larger than the output ordinarily transmitted from said motor but smaller than the impact force generated between said motor and said wheels. PA1 a housing; PA1 a motor mounted on said housing and having a rotary shaft; PA1 an output shaft connected .to a steering mechanism for steering wheels; PA1 a reducing mechanism connected to said output shaft, also provided with an input member and adapted to transmit power in reducing manner from said input shaft to said output shaft; and PA1 power transmission means provided between the rotary shaft of said motor and the input member of said reducing mechanism, and adapted to effect power transmission; PA1 wherein said power transmission means includes a friction plate which is incapable of relative rotation to either of said rotary shaft and said input member but is axially movable relative hereto, a receiving member so mounted on the other of said rotary shaft and said input member as to be incapable of relative rotation thereto, and a pressure member for pressing said friction plate to said receiving member; PA1 the power transmission is achieved by the frictional force generated between said frictional plate and said receiving member; and PA1 said frictional force is so selected as to be larger than the output ordinarily transmitted from said motor but smaller than the impact force generated between said motor and said wheels.
However, in such conventional electric power steering devices;